Nuclear magnetic resonance (NMR) is a resonance phenomenon of energy of a nuclear spin (magnetic moment) generated when an electromagnetic wave is applied to a sample placed in a strong magnetic field. A nuclear magnetic resonance apparatus (NMR apparatus) is a machine which analyzes a structure of a sample using such a resonance phenomenon. As magnetic field strength increases, sensitivity and resolution of an NMR signal increase, and accordingly, a magnetic field generating device for generating a strong magnetic field is included in the NMR apparatus.
As the magnetic field generating device which generates a strong magnetic field, a superconducting magnetic field generating device which generates a magnetic field by magnetizing a superconductor has been disclosed. As the superconductor included in the superconducting magnetic field generating device, it is preferable to use a high-temperature superconductor which has a high superconducting transition temperature and comparatively easily performs cooling.
A sample is placed in a magnetic field when analyzing a molecular structure of a sample using the NMR apparatus. At that time, when variation in magnetic field strength is great depending on location, the obtained NMR spectrum becomes broad and it is difficult to suitably identify the molecular structure of a sample. Accordingly, the superconducting magnetic field generating device used in the NMR apparatus is preferably configured so as to generate a strong magnetic field and to form a magnetic field (homogeneous magnetic field) having a uniform magnetic field strength in a measurement space of the sample.
Uniformity of a magnetic field equal to or smaller than 1 ppm is required for high resolution NMR measurement. In general, a magnetic field having extremely high uniformity which is equal to or smaller than 1 ppm can be achieved by adding a plurality of shim coils (magnetic field correcting coils) to the superconducting magnetic field generating device. That is, the uniformity of the magnetic field generated by the superconducting magnetic field generating device should be at least correctable ppm order using the shim coils.
The superconductor included in the superconducting magnetic field generating device used in the NMR apparatus is, for example, formed in a cylindrical shape. In this case, the magnetic field (applied magnetic field) is applied to the superconductor by an external magnetic field generating device, so that a magnetic field, through which a magnetic flux passes in an axial direction, is generated in an inner circumferential space (bore) of a cylindrical superconductor. The superconductor is cooled to a temperature equal to or lower than the superconducting transition temperature, while the magnetic field is applied thereto. After completing cooling, the applied magnetic field generated by the external magnetic field generating device is removed. By doing so, the superconductor is magnetized so as to maintain the applied magnetic field and a supercurrent is induced into the superconductor. By flowing the supercurrent into the superconductor as described above, a magnetic field (trapped magnetic field), through which a magnetic flux passes in an axial direction, is formed in the bore of the superconductor. A space (room temperature bore space) for placing a sample is formed in the bore of the superconductor in which the trapped magnetic field is formed. A weak electromagnetic wave is emitted from the sample, by applying an electromagnetic wave to the sample placed in the room temperature bore space. By detecting this electromagnetic wave, the NMR spectrum is obtained.
As shown in FIG. 21, in order to improve accuracy of analysis of the sample, the trapped magnetic field formed in the bore of the cylindrical superconductor may have an axisymmetric magnetic field strength distribution (that is, the same magnetic field strength distribution in any direction orthogonal to a central axis of the cylindrical superconductor) and may have a uniform magnetic field strength in the central portion (space where the sample is placed). In order to obtain such a trapped magnetic field, the supercurrent flowing through the magnetized superconductor should be a circular current flowing in a circumferential direction by setting the central axis of the cylindrical superconductor as a center. That is, it is necessary to form a ring-shaped and concentric circular supercurrent loop shown in a cross section which is orthogonal to the axial direction of the superconductor in the superconductor, in order to form the trapped magnetic field having the axisymmetric magnetic field strength distribution and uniform magnetic field strength in the central portion in the bore of the superconductor. However, when a material structure or superconducting characteristics of the superconductor used are not uniform, the supercurrent loop is formed in a deformed shape which is different from the concentric circular shape. That is, the supercurrent loop is disordered. When the supercurrent loop is disordered, the axial symmetry of the magnetic field in the bore of the superconductor breaks down and the uniformity deteriorates, and accordingly, it is difficult to form the trapped magnetic field having the axisymmetric magnetic field strength distribution and uniform magnetic field strength in the central portion in the bore of the superconductor. Therefore, the material structure or the superconducting characteristics of the superconductor are preferably uniform.
As the high-temperature superconductor, an RE—Ba—Cu—O based (RE is a rare earth element containing Y) superconducting bulk manufactured by a fusion method is well known. However, such a superconducting bulk has the following properties which cause disordering of the supercurrent loop (hereinafter, referred to as non-uniformity characteristics).
(1) Since the superconducting bulk is manufactured by allowing single crystal growth from a seed crystal loaded on the superconductor, the superconducting bulk has a crystal growth boundary. For example, when the superconducting bulk is manufactured by allowing crystal growth from a seed crystal loaded so that a c surface of a crystal structure comes into contact with the upper portion of the superconductor, the seed crystal grows in a cross shape in the center in a top view, and accordingly, a cross-shaped crystal growth boundary is formed in the superconducting bulk. When the superconducting bulk formed as described above is magnetized, the shape of the supercurrent loop formed in the superconducting bulk is a square which connects adjacent crystal growth boundaries. That is, the supercurrent loop is disordered due to the supercurrent flowing thus expanding outside of the crystal growth boundary portion, and the concentric circular supercurrent loop is not formed. As a result, the axial symmetry of the trapped magnetic field formed in the bore of the superconductor breaks down and the uniformity deteriorates.
(2) The superconducting bulk as the high-temperature superconductor has a structure in which a non-superconducting phase is finely dispersed in a superconducting phase of the single crystal. The non-superconducting phase forms a pinning point for trapping a strong magnetic field, but there are variations in a size or distribution of the non-superconducting phase, and the supercurrent loop formed in the superconducting bulk is disordered due to such variations.
(3) The superconducting bulk has a material structure with non-uniform characteristics such as vacancies, unnecessary precipitates, presence or absence of microcracks, or disordered crystallinity. The supercurrent loop formed in the superconducting bulk is disordered due to such a material structure with non-uniform characteristics.
(4) The superconducting bulk has local variations in superconducting characteristics (superconducting transition temperature Tc, critical current density Jc, and the like). The supercurrent loop is also disordered due to this.
Accordingly, when using the superconducting bulk in the high-temperature superconductor included in the superconducting magnetic field generating device, the supercurrent loop is disordered due to the non-uniformity characteristics described above, and thus, it is difficult to form the uniform trapped magnetic field in the bore of the cylindrical superconductor, even when the applied magnetic field is uniform.
JP 2008-034692A (Reference 1) discloses a superconducting magnetic field generating device including a cylindrical superconductor which is configured by coaxially arranging superconducting bulks having a cylindrical shape and small magnetic susceptibility on both end surfaces of a superconducting bulk having a cylindrical shape and large magnetic susceptibility. According to the superconducting magnetic field generating device disclosed in Reference 1, it is possible to form a trapped magnetic field having uniform magnetic field strength in an axial direction of the superconductor in the bore of the superconductor, by designing the superconducting bulks so that the magnetic susceptibility and the shape of the superconducting bulks satisfy certain conditions.
JP 2009-156719A (Reference 2) discloses a superconducting magnetic field generating device including a correction coil which is disposed around a circumference of a superconductor formed of cylindrical superconducting bulks. According to the superconducting magnetic field generating device disclosed in Reference 2, it is possible to form a trapped magnetic field having uniform magnetic field strength in an axial direction of the superconductor in the bore of the superconductor, by correcting the applied magnetic field using the correction coil when performing magnetizing by applying a magnetic field to the superconductor.
JP 2014-053479A (Reference 3) discloses a superconducting magnetic field generating device including a cylindrical superconductor which is formed so that an inner diameter of the central portion in an axial direction is greater than an inner diameter of an end portion. According to the superconducting magnetic field generating device disclosed in Reference 3, a magnetic field flows into the bore of the superconductor so as to offset a non-uniform magnetic field generated by magnetization of the superconductor, by setting the inner diameter of the central portion of the cylindrical superconductor in the axial direction to be greater than the inner diameter of the end portion thereof. By removing the non-uniform magnetic field as described above, it is possible to form the trapped magnetic field having uniform magnetic field strength in the superconductor in the axial direction in the bore of the superconductor.
According to the superconducting magnetic field generating devices disclosed in Reference 1, Reference 2, and Reference 3, it is possible to increase the uniformity in the magnetic field strength in the bore of the superconductor in the axial direction, but in principle, it is difficult to increase the uniformity in the magnetic field strength in a circumferential direction. Accordingly, it is difficult to set the magnetic field strength in the bore of the superconductor, particularly in the measurement space (room temperature bore space) of the sample formed in the central portion of the bore to be sufficiently uniform.